Macbeth is the first person to see King Duncan dead after he murders him in his sleep. Lady Macbeth later discovers the body as well.
Macduff appears in several key scenes in Macbeth, including when he discovers Duncan's murder in Act 2, Scene 3, when he flees to England to seek help in Act 2, Scene 3, and when he learns about the murders of his family in Act 4, Scene 3. He plays a pivotal role in ultimately overthrowing Macbeth in the final act of the play.
The Prophecy says Macbeth will be king. Duncan is king. Macbeth cannot be king while Duncan is king. Duncan will not be king when he is dead. Therefore, make Duncan dead.
Macduff discovers that Macbeth is responsible for the murder of King Duncan. He becomes one of Macbeth's main adversaries and plays a crucial role in bringing about Macbeth's downfall.
Macduff and Lennox appeared at Inverness Castle, which was Macbeth's home. Macduff indicated that the King had wanted him to stop by early. Macbeth led Macduff to the room where the King was staying. But only Macduff went inside. Macduff therefore was first to discover the murdered King Duncan.
Macbeth kills Duncan's grooms after Macduff discovers the body.
The witches show Macbeth a procession of Banquo's descendants as kings.
"I" said Macbeth, "with the servants' daggers. I killed Macbeth."
After everyone found out Duncan was dead, Macbeth claimed that it was the guards who had slain King Duncan, as he made sure to smear the blood on their drunk, passed-out bodies.
After Duncan's boy is discovered dead, Macbeth kills the King's guards out of rage and panic. Lady Macbeth tries to maintain control and cover up their involvement in the murder. Macbeth is eventually crowned King of Scotland.
Initially Duncan is the king in Macbeth, however Macbeth later kills him and becomes king himself.
Macbeth gets beheaded which stops him from being the king any more. Malcolm takes over as king.